This invention relates to a vacuum circuit breaker, more particularly a longitudinal or axial field type vacuum switch utilizing contacts made of an Ag-WC alloy.
A vacuum circuit breaker generally comprises a vacuum vessel made up of an insulating casing and end plates closing the opposite ends thereof, and a pair of separable electrodes disposed in the vacuum vessel. The electrode utilized in the conventional vacuum circuit breaker comprises a central contact 1 and a spiral electrode 2 surrounding the contact 1 and provided with a plurality of spiral grooves 2.sub.a, as shown in FIG. 1. With this construction, an electric arc struck at the time of circuit interruption is driven by rotating magnetic flux thus improving the ability of interrupting large currents. More particularly, the arc is rotated around the electrode 2 for avoiding local heating thereof thus enabling satisfactory interruption of large currents.
To enable interruption of large currents, it is necessary to transfer the arc struck at the contact 1 to the electrode 2. To this end, it is necessary to use materials having a small arc voltage difference for the contact 1 and the electrode 2. Since material having high electroconductivity has excellent interruption performance, copper has generally been used for electrode 2. On the other hand, alloys containing copper as the major ingredient has been used for the contact 1, and bismuth has been added to the copper alloy for the purpose of preventing fusion of the conacts which occurs when large short circuiting current flows therethrough.
The Cu-Bi contact material accompanies such problem that the bismuth evaporates selectively when it is subjected to high temprature arc thereby increasing the current chopping level during the use of the vacuum circuit breaker, which results in the creation of switching surges. Among copper rich contact alloys are also included Cu-Al alloys and similar alloys. These prior art copper alloys have been developed for the purpose of improving interrupting performance, anti-fusing characteristic and voltage resistant characteristic but are not always satisfactory for preventing switching surge.
Ag-WC type contact materials have been used as low surge contact materials. Since these materials have excellent low surge characteristic, they have been used abundantly for vacuum switches not requiring large interrupting capacity, but these materials involve many problems to be solved for use in vacuum switches for interrupting large currents.
Vacuum circuit breakers are generally used in electric power systems having low surge impedances, in which the chopping level does not present any serious problem so that above described copper rich contact materials could be used. However, in recent years, the electric power systems are becoming complicated and since a chance of interrupting inductive loads such as large capacity induction motors with vacuum switches is increasing it is necessary to solve the problem of switching surge depending upon the type of loads. In such case, injurious surges have been alleviated by connecting surge absorbers in circuit with the vacuum circuit interrupters.
As is well known in the art, the current interruption phenomenon of a vacuum circuit breaker utilizes diffusion in vacuum of metal vapor generated in a large quantity by arc struck at the time of current interruption. Concurrently with its generation, the metal vapor diffuses in a surrounding vacuum space to deposit and condense on an arc shield or low temperature portions of the electrodes whereas the vapor pressure at the arc is high. At a current zero, the energy supplied to the arc decreases to zero and the interrupting performance of the vacuum circuit breaker is greatly governed by the metal vapor remaining at the time of current zero. After the current zero the remaining metal vapor diffuses at a high speed to decrease the vapor pressure. However, during this interval, since recovering voltage is applied across the electrodes, satisfactory current interruption cannot be realized unless the mean free path of the metal vapor exceeds the length of the electrode gap and the vapor pressure is decreased sufficiently to withstand the rise in the recovering voltage.
The variation in the density of the metal vapor after current zero is expressed by the following equations: ##EQU1## where: n.sub.o : initial density of the metal vapor at a current zero;
erf: error function; PA0 R: radius of electrode; PA0 M: mass of a metal atom; PA0 K: Boltzman constant; PA0 T: temperature of metal atoms; PA0 L: gap length between electrodes. PA0 I.sub.rms : effective value of the current interrupted PA0 E: evaporation coefficient
The initial value n.sub.o is given by the following equation: ##EQU2## where .omega.: 2.pi.f, f: frequency
As can be noted from these theoretical equations, the interruption performance of a vacuum circuit breaker depends upon not only the geometrical construction of the electrodes but also the characteristics of the electrode material.
We have found that the general theory described above cannot be supplied to the Ag-WC contact material utilized in this invention. More particularly, the Ag-WC contact material is a sintered alloy which is prepared by press molding a power of WC, sintering the press molded back and then impregnating Ag so that the interrupting performance is largely governed by Ag having lower melting point than WC. Accordingly, it is impossible to consider the Ag-WC alloy as the source of metal vapor remaining after current zero. For this reason, stable arc voltage of Ag-WC contacts is about 20 to 30 volts which is lower than the arc voltage of 40 to 80 volts of copper rich contact materials, for example Cu-Bi alloys and the stable range of the arc is narrow. As a result of investigation concerning arc of Ag-WC contacts, we have found that in a current interruption limit range a large number of red heat metal particles fly about the arc. It is considered that this phenomenon is caused by the fact that the arc consists essentially of ionized vapor of Ag whereas sputtering of red heat metal particles is caused by the destruction of the bonding of the particles of WC. For this reason, the limit of the interruption performance cannot be determined by the prior art theory in which only the amount of remaining metal vapor and diffusion thereof have been taken into consideration. Rather, the limit should be based on the destruction of the bond between WC particles. For this reason, as shown in FIG. 2, the interrupting performance of Ag-WC type contact is much inferior than that of copper or copper rich alloy thus resulting in a difference in the interrupting mechanism.
On the other hand Ag-WC type contact materials have excellent low surge characteristic so that these materials have been exclusively used for vacuum circuit interrupters having a rating of less than 8 kA, not requiring high interrupting performance.
In recent years, electric power systems become complicated and their capacities become large so that vacuum circuit breakers connected in series with such inductive load as motors are required to have a low surge characteristic.
Since a vacuum circuit interrupter is required to have a large interrupting capacity, as above described a copper electrode with spiral grooves has been used, and at the center of the electrode is mounted a contact made of copper incorporated with a small quantity of Bi or the like which is a welding preventing material. As is well known in the art, the principle of circuit interruption of such spiral electrode is to transfer electric arc struck between separated contacts to the spiral electrode by magnetic force thus causing the arc to rotate by rotating magnetic force created by the spiral grooves. Consequently, local heating of the electrode is prevented and metal vapor remaining at the time of current zero is decreased thereby improving the interruption performance. In order to improve the interruption performance with this electrode construction it is absolutely necessary to transfer the arc onto the electrode. To this end, it is necessary to construct the contact and the electrode with materials having substantially the same arc voltage. Thus contact materials of Cu-Bi type or Cu-Te type are suitable for spiral electrodes in view of their arc voltages. On the other hand, Ag-WC type contact material cannot improve the interruption performance because its arc voltage is lower than that of Cu as above pointed out and is extremely difficult to transfer arc struck between contacts to the electrode.
Accordingly, Ag-WC type contact material cannot be used in vacuum circuit breakers of large interrupting capacity because its interruption phenomenon is different from those of the other contact materials and because it cannot be applied to a spiral electrode owing to the difference in arc voltages although Ag-WC type contact material has an excellent low surge characteristic.
We have investigated the degree of improvement of the interruption performance caused by the use of Ag-WC together with so-called longitudinal magnetic field electrode and found that the interruption performance could not be improved as expected as shown by FIG. 3. Because an appropriate intensity of 30 gausses/I.sub.B determined by experiment by taking into consideration the quantity of metal vapor remaining at the time of current zero cannot be applied to contact materials having different interruption phenomena, where I.sub.B represents the crest value in kA of a rated current to be interrupted.
We have also made various experiments on the longitudinal magnetic field electrode to know that for what reason can Ag-WC type contact material having excellent low surge characteristic interrupt large current. The result is shown in FIG. 4 from which it can be noted that when the intensity of the magnetic field is increased above 50 gausses/I.sub.B, the current interruption performance can be improved greatly. The result of experiment revealed that this was caused by the fact that the limit of destroying the bonds between WC particles was improved by increasing the intensity of the magnetic field.
As above described, we have found an optimum field condition for a case where Ag-WC contact material is combined with a longitudinal magnetic field electrode, and succeeded to provide a vacuum circuit breaker of large interruption capacity and excellent low surge characteristic.
Chopping phenomenon means a phenomenon in which current is abruptly interrupted before a current zero point caused by instability of small current arc. As expressed by the following equation (5), surge voltage caused by this phenomenon is a product of chopping current I.sub.c and a surge impedance Z.sub.s and damages insulation of electric machines and apparatus: ##EQU3## where L represents line inductance and C line capacitance which are the surge constants of an external circuit. Accordingly, in order to decrease the surge voltage it is necessary to lower the chopping level. Although the theory of the chopping phenomenon is not yet established well, it has been proposed to use such material having high vapor pressure as Bi and Sb as contact material as a method of decreasing unstable region of small current arc for causing small current arc to persist thus lowering the chopping level. Cu-Bi type contact material is based on this concept and has been used to some extent but as the melting point of Bi is lower than that of Cu and the vapor pressure is higher, Bi is readily vaporized by the arc thus manifesting a tendency of selective evaporation. For this reason, when current interruption is repeated many times the quantity of Bi in the contact becomes deficient thus increasing the chopping current.
The mechanism of chopping phenomenon is different for Ag-WC contact materials, that is inspite of the use of Ag having much lower vapor pressure than Bi, the chopping current of this material is low. It is considered that the thermionic radiation of WC contributes to the stabilization of small current arc. The chopping level of the Ag-WC material does not increase but remains at a stable low level even after many times of interruption.